Transdermal patches store drugs in gel reservoirs or polymer matrices and gradually release the drugs via a rate controlling membrane when applied to the skin of a patient.
In this specification, the term “drug” is used as a shorthand for any formulation that needs to be introduced into the body of a human or animal patient, whether for therapeutic or non-therapeutic (e.g. cosmetic) purposes. It may be in a form that allows it to flow under modest pressure from the reservoir to the skin, and may be a liquid, a gel, a semi-solid, a suspension of solid particulates, or a suspension of vesicles such as liposomes, niosomes, transfersomes, or elastic type vesicles.
Transdermal patches have been in use since the 1970s for the delivery of drug to a patient via the skin. This has a number of distinct clinical advantages over other means of drug delivery including enhanced patient compliance and a significant reduction in side effects due to a combination of reduced dosage requirements and by-passing of the first-pass metabolism. Patches have been developed consisting of polymer membranes into which drug is loaded, drug loaded adhesives, reservoir matrices containing saturated solutions of drug which diffuses via a rate limiting membrane and gels in which drug is incorporated. Published patent application WO 2005/120471 discloses a patch in which an extensor mechanism is actuated—typically by a microelectromechanical control—to stretch and deform the reservoir, thereby actively expelling the drug from it in a controlled manner.
Transdermal and in particular topical drug delivery has also been attained using various aqueous, gel, and suspension type formulations that are applied directly to the skin. In particular, various vesicle and nanoparticle formulations have been developed that allow a wider range of molecules to permeate the skin via the stratum corneum, which otherwise poses a formidable barrier to the ingress of foreign materials. Indeed there are numerous examples of formulations in aqueous and gel form that are approved by regulatory agencies governing the registration of medicinal products around the world. A fundamental requirement of these agencies however is to ensure the efficacy is not compromised through inadequate dosing; thus ensuring the amount of medication applied each time is controlled and preferably operator independent.
Vesicle and suspension type formulations cannot generally permeate rate controlling membranes due to their physical size and physico-chemical properties. Furthermore it would be preferred to avoid the need for a rate controlling membrane, so as to reduce the costs and timelines associated with validating the product in terms of compatibility with adhesives and the rate controlling membrane. Furthermore it would be preferable to be able to dose a specific volume of drug formulation, thus concentration of drug on to the skin, independent of operator intervention. It would also be preferable to be able to store discrete doses of the required drug in discrete packages until the point of administration, and allow it to be applied to the skin in such a manner that the formulation is not exposed to the atmosphere or prone to being disrupted in any way by the user, e.g., by being rubbed on to clothing or other objects.